The present invention relates to a glass ceramic electric cooker hob with various heaters and with detection of a metal object by means of the generation of a high-frequency magnetic field within an inductive ring of each heater and an electronic circuit for measuring and evaluating the resultant voltage signals.
Monitoring the power of a glass ceramic electric cooker hob permits better distribution of the power among the various cooker heaters and a saving of the energy consumed when the power control is either activated automatically, depending on the presence of the pan, or else the power is regulated in accordance with the heater area covered by the pan.
Devices are known for detecting the presence of a pan on a glass ceramic cooker hob, comprising means for generating an electric field in a conductor loop, situated below the glass ceramic hot plate and covering the heating resistors, and an electronic circuit for evaluating the variation in the voltage at the ends of the loop as a result of the presence of a metal pan and for switching the heater power control on or off, depending on the proximity and size of the pan on the heated area.
Sensor circuit are also known for detecting a pan on a glass ceramic cooker hob, with a sensor loop extended below the area of a heater, comprising a circuit for conditioning the voltage signal obtained from the ends of a sensor loop and a reference signal generator circuit, which has predetermined value in both extreme conditions of the sensor loop, namely in the absence of a pan and with a pan covering the heated area entirely.
The object of the present invention is an electronic circuit with an inductive type sensor for detecting a metal pan on the heated area of an electric cooker hob, as defined in claim 1.
The invention overcomes the problems of improving the simplicity, economy and precision of the detector circuit and sensor, since the latter comprises a single conductor loop of simple layout superimposed on each hob heater, a magnetic field for detection is generated by the actual sensor loop, and a high-frequency electric current for generating the magnetic field comes from a microcontroller which extracts pulse trains from the generator circuit to prevent electromagnetic disturbances in the supply network. The proximity of the metal pan is measured through the variation in voltage at the ends of the loop, unaffected by the length of the connection cables, and it is due to the induced eddy currents in the pan, which cause a reduction in the inductance of the sensor loop.
The sensor comprises a single conductor loop of simple layout, superimposed on each cooker hob heater, the magnetic field being generated under the pan, which is affected by the area of the loop covered by the pan since the sensor circuit is located below the cooker hob control panel, the differences in humidity and ambient temperature, the latter ranging from 0xc2x0 C. to 125xc2x0 C., lead to deviations in the result of the measurement on different measuring occasions.
An objective sought by the invention is measurement with low frequency signals in order to simplify the parts of the sensor measuring circuit. In a first sensor circuit embodiment the voltage signal obtained from the measurement is amplified and then demodulated to extract its envelope, prior to its evaluation. In a second sensor circuit embodiment, its subsequent differential amplification is achieved at low frequency, high input voltage values being obtained at the microcontroller for their comparison.
An additional objective sought by the invention is to prevent not only the influence of the actual impedance of the sensor on the result of the measurement, thereby increasing the sensitivity of the sensor circuit, which is affected by the length of the sensor loop supply cables, but also the influence of the ambient temperature of the sensor circuit. The ambient temperature close to the sensor circuit, since this is located below the cooker hob control panel, may vary from 0xc2x0 to 125xc2x0 C. on different occasions of measurement.
In the first embodiment according to the present invention, the measurement evaluating circuit has a circuit part built-in so as to offset the deviations in the measurement that are usually produced by differences in humidity and ambient temperature. The measuring voltage is taken between both ends of the sensor loop, unaffected by the supply cables.
In the second embodiment according to the present invention is used at least one reference signal generated in normal temperature and humidity conditions in the sensor circuit during detection by way of an additional inductive circuit, which has an impedance representative of all the heaters. It thereby achieves not only an improvement in the sensitivity and resolution of the electrical measurement representative of pan detection, but also the simplification of the cooker electrical wiring, as it uses a single supply cable for generating the magnetic field in the sensor loop and for its measurement, the measuring voltage signal being taken from the free end of the single cable.
A numerical value representative of each heater is recorded at the microcontroller for comparison with a numerical value obtained from measurement. The limit numerical value for the actuation of heating power is pre-set by means of a test with a pan of appropriate size for the area of the heater detected. The numerical value of the measurement of each heater is obtained by means of a calculation that relates the voltage signal measured to the at least one reference signal transmitted.